Due to the steep rise of fossil energy price in recent years, required has been the system which can generate electric power directly from natural energy. To respond such requirement, it has been proposed or put to practical use a solar cell using Si in a single crystal, a polycrystalline state, or an amorphous state, a solar cell made of a compound of GaAs or CIGS (namely, a semiconductor material containing copper (Cu), indium (In), gallium (Ga) and Selenium (Se)), or a dye sensitized photoelectric conversion element (Gratzel cell).
However, the cost of electric power generation with these solar cells is still higher than the price of electricity generated and transmitted using the fossil fuel, and it has been an obstacle for the spread of solar cells. Moreover, it has been needed to use a heavy glass as a substrate, and a reinforcement work has been required at the time of installation. This has also been a cause which increases the cost of electric power generation.
Under such circumstances, it was proposed a bulk heterojunction-type photoelectric conversion element as a solar cell which can achieve lower electric generating cost than the electric generating cost using a fossil fuel. In this photoelectric conversion element, there is a photoelectric conversion layer, in which an electron donor layer (p-type semiconductor layer) and an electron acceptor layer (n-type semiconductor layer) are mixed, sandwiched between an anode and cathode (for example, refer to Non-patent document 1). In this solar cell, an efficiency of 5% or more has been reported.
In these bulk heterojunction-type solar cells, the composing members are formed with a coating process except for an anode and a cathode. Therefore, it is expected that high-speed and low cost production is possible, and the problems of the above-mentioned electric power generation cost may be overcome. Furthermore, unlike the above-mentioned Si system solar cell, the compound semiconductor system solar cell, and the dye-sensitized solar cell, there is no high temperature process above 160° C. Therefore, it is expected that formation of a cell on a low cost and lightweight plastic substrate is also possible.
On the other hand, durability is also required for a solar cell. However, the durability of an organic thin film solar battery is still insufficient, and the improvement is still expected.
In view of the foregoing problems, an organic thin film solar cell having a so called reverse layer structure in which elections are taken out from the transparent electrode side and positive holes are taken out from the side of the stable metal electrode having a deep work function by laminating the element in a reversed order to the lamination order of a usual organic thin film solar cell (refer to Patent Document 1 and Non-Patent document 2).
It has been disclosed that, by using such a construction, a notably improved durability of a solar cell can be attained by suppressing the deterioration originated from the electrode, since there is no need to use a metal electrode having a smaller work function, which is unstable and easy to be oxidized.
The photoelectric conversion efficiency (η) of an organic thin film solar cell is expressed by the following formula. It is necessary to improve each of open circuit voltage (Voc), a short-circuit current density (Jsc), and a fill factor (FF) for improvement in the efficiency of a solar cell.Photoelectric conversion efficiency (%)=open circuit voltage (V)×short circuit current density (mA/in2)×fill factor (FF)
However, in the reverse layer type organic thin film solar cell as disclosed by such as the above mentioned Patent Document 1 and Non-Patent document 2, the fill factor was as low as 0.52 which had been a problem to improve the efficiency.
The FF value has a correlation with the electrical property of a solar cell as a diode, and the following properties are desired to improve the FF value.
1) The resistance is large when a reverse bias is applied.                (The parallel resistance is large.)        
2) The resistance is small when a forward bias is applied.                (The series resistance is small.)        
As a useful method to increase the resistance when a reverse bias is applied, for both, a method to provide a concentration gradient has been considered, namely, to provide a structure in which a p-type organic semiconductor material is eccentrically located near the electrode from which positive holes are taken out (anode) and an n-type organic semiconductor material is eccentrically located near the electrode from which electrons are taken out (cathode).
In such a structure, a low resistance diode can be obtained since occurrence of charge injection barrier is avoided. Such charge injection barrier may occur when an n-type organic semiconductor material becomes in contact with the anode or when a p-type organic semiconductor material becomes in contact with the cathode, which tends to occur in a usual bulk heterojunction layer. In contrast, when a reverse bias is applied, the region where an n-type organic semiconductor material is in contact with the anode or the region where a p-type organic semiconductor material is in contact with the cathode gives a region where leak tends to occur. Accordingly, the structure in which aforementioned concentration gradient is formed can provide a larger resistance when a reverse bias is applied, in the same way.
In a forward layer structure, a method as disclosed in Non-Patent Document 3 has been disclosed as a method to form a bulk heterojunction layer having an ideal concentration gradient.
Namely, it is designed so that the n-type organic semiconductor material is spontaneously located eccentrically near the cathode by mixing a fullerene derivative which has an alkyl fluoride group exhibiting a very low surface energy. According to such a method, a fill factor of 0.70 has been obtained.
However, in the method of Non-Patent Document 3, it is necessary to use a metal having a small work function as an electrode. Accordingly, a problem that the durability is low has been left unsolved. Also, another problem has been left unresolved, namely, even when this composition is used in a reverse layer structure, a high photoelectric conversion efficiency cannot be obtained since the preferred direction of the concentration gradient cannot be obtained.
Also, for further improvement in the fill factor, it is necessary to lower the resistance while a forward bias is applied (a series resistance). This problem has correlation with the mobility in the bulk heterojunction layer. It is said that the carrier in an organic semiconductor material is conducted in the stacking direction of the π planes. In Non-Patent Document 4, it is disclosed that the guideline to obtain a high photoelectric conversion efficiency is that the π planes are stacked in parallel with the electrode in a forward layer structure.
However, there has been no guideline with respect to the molecular design to stack the π planes in parallel with the electrode plane. Accordingly, a proposal of a means to stack the π planes in parallel with the electrode plane has been desired.
On the other hand, in Patent Document 2, a π-conjugated compound incorporated with an alkyl fluoride group is used in a forward layer structure as an n-type organic semiconductor material exhibiting an excellent electron conducting property.
However, such a material has been difficult to use in a reverse layer structure since the preferred direction of the concentration gradient becomes inverse in a reverse layer structure. Thus, a material which enables spontaneous formulation of concentration gradient even in a reverse layer structure in the same way has been desired.
In addition, in Patent Document 2, a photoelectric conversion element containing P3HT and fluorine-containing semiconductor material has been disclosed. However, the fluorine-containing semiconductor material is used as an n-type organic semiconductor material as well as the element construction is a forward layer structure, whereby no idea to improve the fill factor of reverse layer structure has been disclosed. Further, the photoelectric conversion efficiency remained low.